(1) Field of the Invention
This invention relates to a tunnel for treating articles packaged in heat shrinkable thermoplastic film. More particularly, this invention relates to a rotatable shrink tunnel in which the packaged article to be treated is fed through the tunnel.
(2) Description of the Prior Art
As the food industry moves toward more convenient products, improved packaging is desired. For example, many food products, especially meats, are packaged in heat shrinkable thermoplastic film. One effective use of heat shrinkable thermoplastic films is in processing facilities where meat is pre-packaged in bulk quantities. When a food product is packaged, even if a vacuum is drawn, there is excess film around the product and the packaged product is heated to shrink the film to conform tightly to the product. Controlling the temperature of the film shrinking is important because the temperature must be high enough to cause the thermoplastic film to shrink yet must not be so high as to adversely affect the food product.
Heated shrink tunnels are normally used for shrinking thermoplastic films. The products packaged in heat shrinkable thermoplastic film are conveyed into the shrink tunnel and subjected to a heat transfer medium such as heated air or heated water. The effect of heat transferred by the medium causes the film to shrink and conform tightly to the products in the package.
Shrink tunnels are generally either of the heated air-type or the heated water-type. A shrink tunnel of the heated air-type is disclosed in U.S. Pat. No. 4,204,379 to Mugnai, which discloses a closed circuit heat shrink tunnel using recirculated heated air blown around the packaged product at high speed as the heat transfer medium. The wrapped packages are individually placed in the shrinking chamber. Other heated air-type shrink tunnels gradually move the wrapped package through the shrink tunnel on a conveyor belt at a relatively low speed where hot air is passed over and around the article. One such device is shown in U.S. Pat. No. 3,778,964 to Rowland.
Air is sometimes used as the heat transfer medium because it is inexpensive and easier to handle in a packaging plant, but it does not transfer heat very well. Additionally, in order to impart the heat to the film, the air temperature must be relatively high bringing the temperature close to the distortion temperature of the film or to the temperature at which the food product will be adversely affected. Another disadvantage of the typical hot air shrink tunnels is that the heat in the air is quickly lost as the air moves across the package causing uneven shrinkage from one portion of the package to the other.
Because air is a relatively poor heat transfer medium, heated water-type shrink tunnels are sometimes used wherein heated water is the transfer medium. For example, a variety of heated water-type shrink tunnels are manufactured by the Cryovac Division of W. R. Grace. However, the use of hot water for this purpose, although being a good heat transfer medium, contributes to high overall energy consumption. Also, when a shrink tunnel is used to package meat in bulk sizes, say for example, in a slaughterhouse, heated water-type shrink tunnels tax the building cooling system. The temperature in the slaughterhouse where the meat is processed is kept at about 50.degree. F. or colder. Water vapor can raise the temperature of the meat processing room above the desired temperature. Additionally, when heat shrinking using heated water as the transfer medium, the heated water is sprayed over the package leaving a tightly packaged product but a wet package which must be dried. These disadvantages are perceived by some as detriments to good packaging.
With the foregoing in mind, it is an object of the present invention to provide a heated shrink tunnel wherein a substantial portion of the heat required to shrink the film is imparted to the package by substantial contact of the surfaces of the package with the heated surfaces of the tunnel, thus helping to impart heat to the film causing it to shrink.
Another object of this invention is to provide a heated shrink tunnel wherein a substantial portion of the heat required to shrink the film is imparted by a heated gas passing evenly across all surfaces of the package, thus helping to impart heat to the film causing it to shrink.
Still another object of this invention is to provide a method for heat shrinking a package of a heat shrinkable thermoplastic film by imparting the heat required to shrink the film from the heat transfer medium and the heated surfaces of the shrink tunnel.
Yet another object of the present invention is to provide a shrink tunnel in an enclosed circulating system in which high gas flow rates may be used to avoid extremely high temperatures or alternatively shorten heating times.
An even further object of this invention is to provide a heated air-type shrink tunnel for heat shrinking an increased number of wrapped packages in a rotating, gravity-fed shrink tunnel.
Other objects, features and advantages will become apparent from the following description.